1. Field of the Disclosure
The present disclosure relates to protection circuits, which may prevent damage on a circuit, and it particularly relates to a protection circuit, which may prevent damage on a circuit due to a reverse connection of a power supply thereto.
2. Description of the Related Art
A vehicle-mounted integrated circuit may include a protection circuit for preventing damage due to reverse connection of a power supply such as a battery (see Japanese Unexamined Patent Application Publication No. 5-152526). FIG. 4 illustrates a circuit configuration of an integrated circuit including such a protection circuit. A protection circuit 2 illustrated in FIG. 4 includes two P-channel type MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) Q3 and Q4 which are serially connected between a power supply terminal VDD and a ground terminal GND.
The MOSFET Q3 may have a source connected to the power supply terminal VDD and a gate connected to the ground terminal GND through a resistance R4. The MOSFET Q4 may have a drain connected to the ground terminal GND and a gate connected to the power supply terminal VDD through a resistance R3. The drain of the MOSFET Q3 is connected to the source of the MOSFET Q4, and a circuit C having a desirable function is connected between the connection point and the ground terminal GND.
When a power supply is forward connected to the integrated circuit (forward connection), a high level potential is supplied from the power supply terminal VDD to the source of the MOSFET Q3, and a low level potential is supplied from the ground terminal GND to the gate of the MOSFET Q3. This turns the MOSFET Q3 on. A low level potential is supplied from the ground terminal GND to the drain of the MOSFET Q4, and a high level potential is supplied from the power supply terminal VDD to the gate of the MOSFET Q4. This turns the MOSFET Q4 off. Thus, a current path is formed from the power supply terminal VDD to the ground terminal GND through the MOSFET Q3 and circuit C, and current Idd in a forward direction is fed to the circuit C.
On the other hand, when a power supply is reverse-connected to the integrated circuit (reverse connection), a low level potential is supplied from the power supply terminal VDD to the source of the MOSFET Q3, and a high level potential is supplied from the ground terminal GND to the gate of the MOSFET Q3. This turns the MOSFET Q3 off. A high level potential is supplied from the ground terminal GND to the drain of the MOSFET Q4, and a low level potential is supplied from the power supply terminal VDD to the gate of the MOSFET Q4. This turns the MOSFET Q4 off. In this case, the MOSFET Q3 is turned off, and the MOSFET Q4 is turned on. As a result, a current path through the circuit C is not formed. In this way, the protection circuit 2 may prevent damage on the circuit C by shutting down a current path through the circuit C upon reverse connection of the power supply.
In an integrated circuit including the protection circuit 2, when consumption current Idd increases because the size of the circuit C increases, for example, an increased amount of current is fed to the MOSFET Q3 to which to the power supply is forward connected. As a result, a voltage drop due to an ON resistance of the MOSFET Q3 increases, and the drain voltage of the MOSFET Q3 decreases. Occurrence of such a voltage drop prevents application of a proper reverse bias to a PN junction present in the protection circuit 2, which facilitates flow of unnecessary current and may damage the integrated circuit including the protection circuit 2.
Japanese Unexamined Patent Application Publication No. 2002-335626 is an example of related art.